As is understood, four motion feed mechanisms (also known as drop feed mechanisms) have been known for a number of years. With this particular type mechanism, movement of the feed dog means is comprised of two components; namely, a height-wise movement known as feed "lift" and a horizontal movement known as feed "advance" or "return". In an industry, such as sewing, wherein it is necessary to handle varying type workpieces, ranging from denim to the lightest lingerie, it has been considered desirable to provide a sewing machine with adjustable "lift" motion. An adjustable "lift" motion is desirable because the sewing of materials of different thicknessess or weight requires different magnitudes of feed lift. That is, heavy material, i.e. denim or wool, requires a greater feed lift than lighter material, i.e. silk.
Many of today's machines have fixed feed lift eccentrics. Accordingly, these machines are limited in use as to the type of materials which can be sewn thereon. In one class of sewing machines it has generally been the practice to employ replaceable eccentrics for varying the feed lift. However, a major drawback associated with the use of replaceable eccentrics is the amount of "down time" that the operator must incur in order to change the feed lift of the machine. The heretofore known feeding mechanisms which employed fixed or replaceable eccentrics lack the advantages of the arrangement according to the present invention which employs a new and unique feed mechanism which is designed to permit precise adjustment of the feed mechanism of the machine. Still another constraint that was necessarily weighed during the formulation of the present invention was to design a mechanism wherein it may be possible to adjust or change the feed lift of the machine without correspondingly affecting the feed advance or stitch length.
Sewing machines are known from U.S. Pat. No. 558,663 and No. 2,725,023 wherein the feed "height" is adjustable. That is, the distance the feed dog extends above the work support of the machine. It is important to note that with this type machine, the feed "lift" is not adjustable, but the orbital path traversed by the feed dogs during the operation of the machine is merely raised relative to the throat plate of the machine. As may readily be appreciated by one skilled in the art, a major drawback with merely adjusting the feed height, so as to compensate for workpieces of varying thickness or weight, is the backwards feeding that is incurred when such adjustment is made. Additionally, the raising of the feed dog results in an increased duration of feed above the throat plate.